Thread: Namibia from Space

Another stunning image from NASA's Earth Observatory. With an average of just 258 millimeters (10.2 inches) of rain per year, Namibia is Africa’s driest country south of the Sahara. Desert runs the length of the Namibian coast, and almost all of that desert is now protected land. The creation of Dorob National Park around Swakopmund and Walvis Bay in December 2010 completed a nearly unbroken chain of coastal parks.



The Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra satellite captured this natural-color image of the Namibian coast on June 26, 2011. This image is rotated so that north is to the right. With the exception of a cloud bank hugging the coast near the Kunene (or Cunene) River, skies are cloud-free. Namibia’s deserts are by no means uniform.

The Kuiseb River, flowing toward the Atlantic Ocean about midway down the coast, marks a dramatic difference in landscape. Rocky plains lie north of the river; to the south lies a sea of sand dunes, which appear orange-brown in this image. In fact, sand dunes often march northward past the Kuiseb River, as this linked satellite image shows.

The river is impermanent, and when it lacks enough water to wash out accumulated sand, dunes can continue their northward trip, shifting over older dunes. The rainy season of 2010–2011 brought sufficient rains to the region to send the Kuiseb all the way to the coast for the first time since the early 1960s.

Even though dunes can be spotted north of the Kuiseb in early 2011, they end more definitely near Swakopmund. Estimated to be 55 million years old, the Namib Desert, which spans much of the Namibian coast, may be the oldest desert in the world, according to the United Nations Environment Programme.

The desert owes much of its continued aridity to wind. National Geographic describes two winds: a hot, dry wind from the east that can raise local temperatures to 100 degrees Fahrenheit (38 degrees Celsius) or higher, and a cool, moist wind from the Atlantic Ocean that occasionally brings much-needed moisture. Rain is rare along the Namibian coast, but it does arrive spectacularly sometimes.

References

Fuller, A. (2011). Namibia’s coastal parks
National Geographic, 219(6), 60–69
United Nations Environment Programme (2008)
Africa: Atlas of Our Changing Environment Division of Early Warning and Assessment, United Nations Environment Programme, Nairobi, Kenya.

NASA image courtesy MODIS Rapid Response Team, Goddard Space Flight Center. Caption by Michon Scott.

The Namib Desert may be the best-known desert in Namibia, but it is not the only one. Eastern Namibia is also dry, though slightly less so. This part of the country holds part of the expansive Kalahari Desert, home to some visually striking dunes.

The Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite captured this image on May 7, 2007. ASTER combines infrared, red, and green wavelengths of light to make false-color images. Thick vegetation lining the Nossob River appears bright red. The rest of the sparsely vegetated landscape appears in shades of purple-brown and burnt orange.

The long, thin, diagonal stripes are linear dunes, which occur along the southwestern margin of the Kalahari Desert. Linear dunes typically form in arid environments with consistent wind patterns. Research conducted in the 1980s and 1990s found that current climatic conditions in this region are not conducive to the formation of an extensive dune system. Wind direction varies too much, and vegetation, although sparse, is abundant enough to inhibit the movement of sand. By drilling into the dunes and examining sediment samples, researchers concluded that the dunes likely formed between 17,000 and 10,000 years ago. Another dune-forming period may have occurred between 28,000 and 23,000 years ago.

Although conditions no longer support the development of new dunes, some sand movement does occur, particularly along the crests. Compared to conditions 10,000 years ago, however, wind speeds are low and vegetation—nurtured by annual precipitation of about 150 to 200 millimeters (6 to 8 inches)—comparatively abundant.

References
Namibian.org. Kalahari. Accessed December 1, 2011.
Nelson, S.A. Wind Action and Deserts. Tulane University. Accessed December 1, 2011.
Stokes, S., Thomas, D.S.G., Shaw, P.A. (1997). New chronological evidence for the nature and timing of linear dune development in the southwest Kalahari Desert. Geomorphology, 20, 81–93.

NASA Earth Observatory image created by Jesse Allen and Robert Simmon, using data provided courtesy of NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team. Caption by Michon Scott.

A new image released by NASA: Linear Dunes in the Caprivi Strip



Credit: NASA Earth Observatory image created by Robert Simmon and Jesse Allen, using Advanced Land Imager data from the NASA EO-1 team. Caption by Michon Scott.

In far northeastern Namibia, there is a skinny stretch of land sandwiched between Angola, Botswana, and Zambia. The Caprivi Strip receives more than 600 millimeters (24 inches) of mean annual rainfall and experiences periodic floods, making it almost moist compared to the much drier parts of the country.

On February 1, 2012, the Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite captured this natural-color image of the Caprivi Strip just north of the Okavango River (visible in the large image). Here the land is striped, as if a giant had dragged a rake over the landscape. Those stripes are linear dunes, and some are more than 100 kilometers (60 miles) long. Their presence suggests much drier conditions in the past.

Dunes generally form from wind-blown sand over many years. One characteristic of linear dunes is that they tend to remain intact long after the dry conditions cease. And because they don’t migrate like marching dunes, linear dunes preserve dirt and rocks that geologists can later use to understand past conditions.

A study published in 2000 sampled dunes throughout the Caprivi region and found that they likely formed under arid conditions between roughly 60,000 and 20,000 years ago. A study in 2003 concluded that dune construction may have been especially pronounced between 36,000 and 28,000 years ago. After the dunes formed, conditions in the Caprivi Strip moistened enough for the dunes to support vegetation—woodlands on the dune ridges, and grasses and shrubs in the valleys between.

Although studies indicate that conditions in this region were drier when the dunes formed, the dune-building periods seem to have been punctuated by humid periods, as indicated by sediments found in nearby caves and ancient lake sediments. About 16,000 years ago, a humid period prompted the filling of Etosha Pan. Now a saltpan, Etosha withered partly due to drying climate, but also because of changes in river routes.


References

Eitel, B., Blümel, W.D., Hüser, K. (2004) Palaeoenvironmental transitions between 22 ka and 8 ka in monsoonally influenced Namibia. Paleoecology of Quaternary Drylands, 102, 167–194.

Thomas, D.S.G., O’Connor, P.W., Bateman, M.D., Shaw, P.A., Stokes, S., Nash, D.J. (2000) Dune activity as a record of late Quaternary aridity in the Northern Kalahari: new evidence from northern Namibia interpreted in the context of regional arid and humid chronologies. Palaeogeography, Palaeoclimatology, Palaeoecology, 156(3–4), 243–259.

Thomas, D.S.G., Brook, G., Shaw, P., Bateman, M., Haberyan, K., Appleton, C., Nash, D., McLaren, S., Davies, F. (2003) Late Pleistocene wetting and drying in the NW Kalahari: an integrated study from the Tsodilo Hills, Botswana. Quaternary International, 104(1), 53–67.

Warren, A. Etosha: From Sand to Sea. PBS: The Living Edens. Accessed April 5, 2012.

Hydrogen Sulfide Emissions off Namibia



Credit: NASA image courtesy Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response Team at NASA GSFC. Caption by Michon Scott.

Pale green patterns tinted the water along the Namibian coast in late February 2012. But unlike other bright hues that occasionally show up in the ocean, these colors didn’t result from a phytoplankton bloom.

Scientists have long known that hydrogen sulfide gas is emitted periodically along the Namibian coast. Ocean currents carry oxygen-poor water to the region, and chemical and biological processes can deplete what little oxygen is available. The sediments in the local seafloor are also rich with organic matter. When organic matter decays in an oxygen-poor environment, hydrogen sulfide emissions can result.

Before the satellite era, residents of the region could detect the hydrogen sulfide emissions thanks to the pervasive rotten-egg smell. But satellites’ “eyes in the sky” have shown just how big and long-lasting the emission events can be.

The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite captured this natural-color image on February 29, 2012. The pale-hued surface waters snake along the shore of the Namib Desert, stretching roughly 150 kilometers (90 miles).

The milky-green colors along Namibia’s coast indicate high concentrations of sulfur and low concentrations of oxygen. Episodes like this aren’t just colorful, they are actually toxic to local marine organisms. Fish die in the low-oxygen water; however, what is deadly for the fish can be good for birds that feed on their carcasses. Likewise, lobsters crawling onto shore to escape the toxic seawater can make meals for locals. And some species of foraminifera—tiny shelled marine organisms—actually thrive in the oxygen-poor sea floor sediments off the Namibian coast.

References

Brüchert, V., Currie, B., Peard, K.R. (2009). Hydrogen sulphide and methane emissions on the central Namibian shelf. Progress in Oceanography, 83, 169–179.

Leiter, C., Altenbach, A.V. (2010). Benthic foraminifera from the diatomaceous mud belt off Namibia: characteristic species for severe anoxia. Paleontologia Electronica, 15(1). 13.2.11A.

Why Files. (2002). The Biggest Burp. Accessed March 1, 2012.

This astronaut photograph shows the white, salt-covered floor in the northwest corner of the Etosha Pan, a great dry lake in northern Namibia. Two rivers, the Ekuma and Oshigambo, transport water from the north down to the Pan. In a relatively rare event shown in this image, water from recent rains has flowed down the larger Ekuma River—which appears as a thin blue line within the light grey-green floodplain—and fills a lobe of the lake with light green water (mid-right). Water has also flowed into a small offshoot dry lake, where it appears a brighter green (upper left).

Other smaller lakes hold red and brown water, a result of the interplay of water depth and resident organisms such as algae. The algae color varies depending on water temperature and salinity. (A similar process is observed in pink and red floodwaters when they pond in Lake Eyre, a mostly dry lake in Australia. In Lake Eyre, researchers know that the color is indeed due to algae growth.)

Typically, little water or sediment reaches the floor of the Etosha Pan because the water seeps into the riverbeds along their courses. The floor of the pan itself is only rarely covered by even a thin sheet of water. In this image, there was enough surface flow to reach the pan, but too little to flow beyond the inlet bay. A prior flood event, when water entered the pan via the Oshigambo River, was documented in astronaut imagery in 2006.

The straight line that crosses the image is the northern fence line of Namibia’s Etosha National Park. This three-meter-high fence keeps wildlife from crossing into the numerous small farms of the relatively densely populated Owambo region of Namibia, north of the pan. The large Etosha lakebed (120 kilometers long, or 75 miles) is at the center of Namibia’s largest wildlife park and a major tourist attraction.

Astronaut photograph ISS030-E-234965 was acquired on December 30, 2011, with a Nikon D2Xs digital camera using a 180 mm lens, and is provided by the ISS Crew Earth Observations experiment and Image Science & Analysis Laboratory, Johnson Space Center. The image was taken by the Expedition 30 crew. It has been cropped and enhanced to improve contrast, and lens artifacts have been removed. The International Space Station Program supports the laboratory as part of the ISS National Lab to help astronauts take pictures of Earth that will be of the greatest value to scientists and the public, and to make those images freely available on the Internet. Additional images taken by astronauts and cosmonauts can be viewed at the NASA/JSC Gateway to Astronaut Photography of Earth. Caption by M. Justin Wilkinson, Jacobs/ESCG at NASA-JSC.

The Orange River serves as part of the border between Namibia and the Republic of South Africa. Along the banks of this river, roughly 100 kilometers (60 miles) inland from where the river empties into the Atlantic Ocean, irrigation projects take advantage of water from the river and soils from the floodplains to grow produce, turning parts of a normally earth-toned landscape emerald green.

The Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite captured this true-color image on February 17, 2010. A network of bright rectangles of varying shades of green contrasts with surroundings of gray, beige, tan, and rust. Immediately south of a large collection of irrigated plots, faint beige circles reveal center-pivot irrigation fields apparently allowed to go fallow.

Namibia is Africa’s most arid country south of the Sahara Desert, according to the United Nations Environment Programme (UNEP). Even though South Africa has a generally temperate climate, 65 percent of the land is too arid to support agriculture. Irrigation projects support agriculture that rainfall alone could not sustain. This irrigation project occurs along a section of the Orange River where the waterway turns north on its general westward path to the sea, and the area isn’t far from the eastern margin of the Namib Desert. Grapes are the primary agricultural product of this area. Thanks to local climatic conditions, grapes from Namibia are often ready for market two to three weeks before those of the main grape-producing regions of South Africa’s Cape.

Just days before ALI acquired this image, the area experienced flooding. Flooding along this section of the Orange River was relatively mild, and floodwaters had receded by the time ALI observed the area.
References

United Nations Environment Programme. (2008). Africa: Atlas of Our Changing Environment. Division of Early Warning and Assessment, United Nations Environment Programme, Nairobi, Kenya.

NASA Earth Observatory image created by Jesse Allen, using EO-1 ALI data provided courtesy of the NASA EO-1 team and the United States Geological Survey. Caption by Michon Scott based on information provided by Guido Van Langenhove, Hydrological Services Namibia, Department of Water Affairs.